The axle beam arranged between left and right wheels of an automobile, the surrounding suspension members, and other automobile chassis parts are repeatedly subjected to an impact load, torsion load, etc. during vehicle operation, so a high strength and also high fatigue characteristics are required. For example, Japanese Patent Publication (A) No. 2001-321846 discloses a hollow structure axle beam obtained by press forming high strength steel pipe into an irregular cross-sectional shape.
The axle beam of this Japanese Patent Publication (A) No. 2001-321846 is improved in fatigue characteristics by press forming high strength steel pipe into an irregular cross-sectional shape, then heating it to a high temperature and then rapidly water cooling it for quenching. For this, however, it is necessary to heat this to a high temperature of the transformation point of the steel material or more, so the heating cost rises and, on top of this, control of the atmosphere is required for preventing oxidation scale or a descaling step is required, so there was the problem that the cost became further higher. Further, in general, high strength steel pipe is poor in shapeability and easily varies in dimensions, so there was the problem that the work efficiency in assembly into the chassis fell.
Note that in the process of production of chassis parts of irregular cross-sections such as an axle beam, the material, that is, the steel pipe, is subjected to strong bending, so the bent parts suffer from large residual stress. Therefore, in the past, when not quenching the material, the residual stress resulted in lower fatigue characteristics. Further, if performing stress-relief annealing to remove the residual stress, there was the problem that the steel material ended up softening, so again the necessary fatigue characteristics could not be secured.